Corneoct/clas.] PELECYPODA. 937 



the posterior very slender, directed backwards. Ligament small, inset. 

 Pallial line simple. 



Diameter Ant. -post., 3-5 mm. ; dorso- ventral, 2-75 mm. : thick- 

 ness, 2 mm. 



Type in my collection. 



Hab. North Island : In a pond at Parnell, Auckland, type (C. 

 Musson) ; Ohaupo ; Otaki Gorge (H. B. Preston). South Island : 

 Heathcote Estuary, in slightly brackish water (H. S.). 



Remark. This species never attains the size of C. novce-zelandi(e, 

 and from young specimens of the latter it may be distinguished by 

 being more inflated and by having the beaks more posteriorly. Most 

 specimens have above and behind a ferruginous coating. 



Fain. UNIONISE, Fleming. 



Animal with labial palpi almost always wider than long ; anal 

 opening usually separated from the superanal ; embryo a glochidium, 

 the soft parts being enclosed in a bivalve shell, and borne in the inner 

 or outer or all four leaves of the branchiae. 



Shell nacreous, with a thick epidermis ; beaks usually sculptured, 

 often showing the remains of the nuclear shell ; ligament opistho- 

 detic ; hinge with or without teeth, though with vestiges of thefn 

 in every genus ; when present, schizodont (heavy, amorphous, and 

 variable teeth, often obscurely divided into subumbonal and lateral 

 elements) and arranged as pseudo-cardinals and laterals ; pallial line 

 usually simple ; prismatic border ordinarily narrow. 



Genus 1. DIPLODON, Spix, 1827. 



Diplodon, Spix, Test. Fluv. Bras., 1827, 33, pi. 26. Type : D. ellipticum, 

 Spix. 



Animal with the marsupium occupying nearly the whole length of 

 the inner branchiae, a few ovules sometimes being found in the outer 

 gills ; branchiae rather large, angular at base, inner much the larger, 

 united their whole length to the abdominal sac ; palpi scarcely pro- 

 jecting posteriorly ; mantle very thin, thickened on the edges ; 

 branchial opening papillose, separated from the smooth anal open- 

 ing by a strong bridge ; superanal opening not closed below. 



Shell elliptical, rounded, elongated or trapezoidal, with rather 

 low beaks which are more or less distinctly radially sculptured, the 

 ridges usually curved and approaching below, with a low or scarcely 

 developed posterior ridge ; surface slightly concentrically sculptured, 

 sometimes broken into fine nodules or corrugations ; epidermis dull. 

 rayless ; hinge with 2 compressed pseudo-cardinals in the right valve, 

 and 1 slender lateral, and 2 compressed pseudo-cardinals in the left 

 valve, one in front of the other, and 2 laterals ; nacre bluish to white, 

 dull, often blotched ; beak cavities shallow ; dorsal scars numerous, 

 forming a row in the beak cavity parallel with the hinge-line. 



